"Limiting access may be impossible," said the bulletin, a copy of which was obtained by Fox News.

The document is distributed to numerous state and federal law enforcement agencies.

The guns, which can be made by downloading blueprints into cutting-edge computers that mold three-dimensional items from melted plastic, "poses public safety risks," the bulletin said.

They are likely beyond the current reach of regulators and threaten gun-control efforts, the bulletin says.

The Homeland Security bulletin singles out Defense Distributed, a nonprofit company started by a University of Texas law student. The company has successfully made and fired a 3D gun whose only metal parts are the bullets and a small firing pin.

About 100,000 plans for a gun called "The Liberator" were downloaded in just a few days before May 3, when a branch of the U.S. State Department told it to stop sharing the file, Fox reports.

A law enforcement source told Fox that the potential problems faced by government authorities involved securing large, high-profile events or those attended by the president, where magnetometers for weapons screening would not pick up a 3D printed gun.

"This is a serious threat," the source told Fox. "These could defeat magnetometers. The only security procedure to catch [the 3D firearms] is a pat-down. Is America ready for pat-downs at every event?"